1. Field of the Invention
This invention relates generally to glass seals for ignition devices, and more particular to spark plugs including glass seals, and methods of forming the same.
2. Related Art
Glass seals are oftentimes used to form a hermetic bond between an electrically conductive component, such as central electrode, and an insulator of an ignition device, for example a spark plug. The glass seal of the spark plug is typically formed by disposing a glass powder in a bore of the insulator, and then subsequently firing the insulator, central electrode, and glass powder together in a furnace. The heat also causes certain components of the glass seal to expand and thus form the hermetic bond between the insulator and central electrode. However, as the glass powder between the central electrode and insulator melts and expands, gas bubbles or gas pores are formed, and those bubbles or pores remain in the glass seal of the finished spark plug, even after the glass seal cools to room temperature. Thus, when the spark plug is used in an internal combustion engine and subjected to a high electric field, the electric field causes the gas contained in the bubbles or pores to become ionized and form corona. The ionized gas generates a cascade of ionized charges which transmits heat to the surrounding solid insulator. A thermal breakdown mechanism occurs, which can create a dielectric breakdown. The effect of this dielectric breakdown caused by the gas is especially pronounced when the bubbles or pores are large, in which case dielectric failure of the insulator can occur. Dielectric punctures through the insulator to the expanded glass seal could potentially result in failure of the spark plug.